1. Field of the Invention
This invention relates generally to a method of quantitatively evaluating the resistivity dispersion of a crystal sample such as a GaAs crystal by measuring the optical absorption image, and more particularly to a method comprising the steps of obtaining the infrared absorption image which is composed of pixels of a sample of such crystal, the infrared rays used having a 1 um wavelength band using an image processor coupled to a computer, obtaining the absorption coefficient (or transmissivity) corresponding to each of the pixels of this absorption image, simultaneously with calculating the mean value and standard deviation value of the absorption coefficients and obtaining quantitatively values related to the resistivity dispersion of the sample on the basis of these values.
2. Description of the Prior Art
To evaluate performance or characteristics of a GaAs crystal as an electronic device, a method of measuring the resistivity by a 3-terminal guard method and a method for measuring the threshold voltage of each of the FET devices made from a GaAs crystal.
It is known, on the other hand, that the resistivity of an undoped GaAs crystal is related with a deep energy level referred to as "EL2". [(1) "Observation of Fine Defects of GaAs Crystal by VE-IRT Process", T. Katsumata et al., Research Report of No. 27th Research Meeting, No. 145 Committee of the Japan Society for the Promotion of Science; (2) "Measurement Method of Distribution of Fine Defects in a Plane of GaAs Wafer", T. Katsumata et al., "Semiconductor World", June, 1985, p. 75. As to the EL2 level, refer to (1) "Deep Energy Level of III-V Compound Semiconductor", T. Ikoma, Semiconductor Research XXIII, (2) "Non-Uniformity Phenomenon around Dislocation in LEC GaAs", S. Miyazawa, papers of the Japan Crystal Growth Society, Vol. 13, No. 2 & No. 3, (1986), p. 144.] It is also known that there is a relationship between the level EL2 and the absorption of infrared rays of a wavelength 1 um band by GaAs crystal. Evaluation of the crystal by the measurement of the optical absorption coefficient is carried out on the basis of these properties.
However, the method of measuring resistivity by the 3-terminal guard method involves the problems that the same processes are required as those through which a device is actually fabricated and at the same time, an extremely long period of time is necessary for the measurement of characteristics. For example, a measurement time of as long as one week is necessary to measure a wafer having a diameter of 2 inches. Furthermore, this method is a destructive method for inspection.
On the other hand, the method using infrared absorption has been employed conventionally for evaluating the GaAs crystal but this method provides only the transmission image by use of a TV camera but does not provide an absolute transmission factor image, this image being obtained by calculating an absolute transmission factor at each pixel.